NATO allies will boost high readiness forces to "well over 300,000" troops as they strengthen their defences in response to Russia's war on Ukraine, alliance chief Jens Stoltenberg said Monday. 

Leaders from the US-led military alliance will meet in Madrid this week for what Stoltenberg said would be a "transformative" summit as it grapples with the fallout of Moscow's invasion of its pro-Western neighbour.

Stoltenberg said allies would bolster some of their battle group deployments along NATO's eastern flank "up to brigade level" -- tactical units of several thousand troops -- and ratchet up high readiness numbers to "well over 300,000".

In addition, more heavy weaponry, including air defence systems, would be shifted forwards and forces pre-assigned to defend specific NATO members on the alliance's exposed eastern edge.

"This constitutes the biggest overhaul of our collective defence and deterrence since the Cold War," Stoltenberg said. 

Stoltenberg did not give further details of the additional high readiness forces or how they could be deployed by the alliance. 

NATO currently has a high readiness force of around 40,000 troops under its command. 

The more than 300,000 troops are expected to form a larger pool that the alliance could tap into in the case of an emergency. 

Stoltenberg also said that leaders would agree to bolster NATO's essential support to embattled Ukraine, whose President Volodymyr Zelensky is set to call in via video link.  

That package would include "substantial deliveries" of gear like secure communications, anti-drone systems and fuel, and help Ukraine over the longer term to pivot to using more advanced NATO-standard arms. 

That support is separate from weaponry that NATO members -- spearheaded by the United States -- are already funnelling to Ukraine, including anti-tank rockets, artillery and air defence to help it hold back Russia's onslaught.